What are the technical challenges for the rise of LED tunnel lighting?

　　The main purpose of general tunnel lighting is  prevent accidents by providing maximum visibility of objects on the road surface of the tunnel, and  enhance safety and comfort for drivers passing through or entering the tunnel. CIE 88-"Guidelines for Lighting of Highway Tunnels and Underground Passages" provides a guideline for tunnel lighting and is the basis of many national regulations. It divides the tunnel in different areas according  the specific requirements of each.

　　First, the entrance section requires very high brightness values ??so that the driver's eyes can adapt  the darker lighting conditions in the tunnel. The length of the entrance section is determined by the maximum speed limit in the tunnel, because a higher speed limit requires a longer entrance section.

　　Secondly, in the transition section, the brightness on the road surface is slowly reduced until it reaches the lighting level of the interior area of ??the tunnel, thereby providing a smooth adaptation from the entrance  the middle section. In the first half of the entrance section, consider that the maintenance facr is 0.67, the lighting is about 150-300 cd/m2, and the internal area is usually limited  2-6 cd/m2.

　　The currently recognized quality parameters of road tunnel lighting include: uniformity of overall and longitudinal road surface lighting (ideally no flicker), lighting of tunnel walls, avoidance of glare, and color temperature and color rendering index. In recent years, with the development of semiconducr technology, LEDs have gradually replaced traditional tunnel sodium lamps. Many national standards have updated the requirements of the above parameters.

　　New LED cusmization standard

　　In addition , for example, the Swiss ASTRA (Bundesamtfür Stra?en) regulations, the Austrian ASFINAG planning manual (PLaPB 800.562) is one of the first standards specifically for LED tunnel lighting systems. This standard introduces clusters in different tunnel lighting categories. It not only specifies the different lighting requirements of each cluster, but also specifies the different distances from 18 meters (cluster standard)  the continuous lighting belt. And it also has different requirements for energy efficiency, lighting color and color rendering. Although color rendering does not seem  be the most important index in tunnel lighting, in view of the increasing number of different signal colors in road tunnels, it is important for drivers  quickly distinguish between yellow and red, or blue and green.

　　Moreover, compared with the traditional lighting system, the advantages of the LED system are also reflected in the true color lighting, high luminous efficiency and efficient lens distribution. The LED system can provide analog dimming inside the tunnel and the entrance area, thereby eliminating the problem of road brightness uniformity caused by turning off the entire group of lamps.

　　The current LED tunnel system has become a smart solution that can communicate the current status of the lamp with the driver. However, it is still under discussion as  where the support and control electronics need  be integrated. Some tunnel markets require intelligent drivers  be installed in lamps, while others require all electronic equipment  be installed in the tunnel maintenance room or control building at the entrance. Both solutions have advantages, the latter reduces maintenance work.

　　Basically, in Germany, Austria and Switzerland, the general characteristics and requirements of tunnel lamps have changed from sodium lamps with CRI 20-30  LEDs with CRI 70 or 80, 4000-4500 K, and even some current projects require efficacy or even more than 110 lm /W, the system whose life span is more than 80,000 hours (the failure rate is less than 10% during this period).

　　Figure 1 Adaptation curve of tunnel brightness

　　Requirements outside of lamp specifications

　　But some requirements, although important, cannot be met by lighting equipment alone. Especially the early LED lamps are more dazzling than traditional lamps, so avoiding glare becomes more and more important. Some regulations already require a threshold increment of 8% or even only 6%, because a lower threshold increment means less glare. The threshold increment is the relative relationship between the brightness of the light curtain and the brightness of the road surface. The lower the brightness of the light curtain or the higher the road surface brightness can reduce glare. The brightness of the light curtain itself depends on the luminous flux and light distribution of the lamp. Lamp manufacturers can develop lamps that can maximize the brightness of the road surface while also providing a relatively low brightness of the light curtain, but there is a key facr that cannot be controlled. This is the brightness coefficient q0 of the road surface, which determines the relationship between illuminance (lx) and brightness (cd/m2). According  the road surface grade, the brightness coefficient usually varies between 0.05 and 0.07. Therefore, q0 can change the brightness value on the road by about 30%. In turn, the luminous flux of the luminaire must be increased by 30%, which will result in higher light curtain brightness and higher threshold increments.

　　Figure 2 Comfortable tunnel lighting cluster

　　Figure 3 Continuous tunnel lighting cluster

　　Efficacy requirements and how  achieve

　　A requirement that is meaningful but not necessarily able  achieve the expected result is the luminous efficiency, that is, lm/W. This value perfectly describes the efficiency of the LED, and higher luminous efficiency means higher energy efficiency. The same is true for all tunnel luminaires with symmetrical light distribution, which is generally applicable  the inner area of ??the tunnel. Since the transmittance values ??of different optical systems on the market are similar, almost close  90%, and the light distribution also provides similar road brightness, the luminous efficiency is the best choice  determine the energy efficiency of the entire lamp. But what about the lamps in the entrance section and transition zone?

　　Usually, these lamps are equipped with reflective elements, which can reflect light  the driver at a very horizontal angle.

　　Compared with luminaires with symmetrical light distribution, reflective luminaires provide better brightness values. Therefore, reflective lamps have always been the choice of tunnel entrance areas in the past, which are both economical and efficient. However, these lamps can only be used in the entrance section and transition areas, because in these areas, the distance between the lamps is small. In the middle section, the reflective lamps will produce o much glare at the illumination distance of 8-15m, and also cause the uniformity of light.

　　As shown in Figure 4, the peak intensity of the reflective luminaire is at a vertical angle above 60°, and more importantly, the light is almost all distributed in one direction. This "bending of light" will cause reflection loss, thereby reducing transmittance, resulting in a decrease in the luminous efficiency of the lamp. For lamps in the entrance section and transition section, the luminous efficiency value is not taken in consideration. Assuming the same lumen input, reflective luminaires with a peak value of 60° and above will produce quite high brightness values ??on the road surface, while those at 55° will be small, although the latter may have higher transmittance values ??and higher luminous efficiency .

　　Figure 4 Reflected light distribution

　　Figure 5 Symmetrical light distribution

　　It should also be noted here that lamps with peak intensity at higher vertical angles do not necessarily produce more glare.

　　Therefore, the specific surface power density is more suitable for describing and requesting the energy efficiency of the tunnel entrance lighting, which includes the known brightness value, the length of the entrance section and the transition zone, and the power consumption of all installed entrance (reflective) lamps.

　　in conclusion